Helmet



FIG. 1 is a front, top, and right side perspective view of the new design for a helmet;

FIG. 2 is a front view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a left side view thereof; and,

FIG. 5 is a bottom view thereof.

The broken lines shown in the drawings illustrate portions of the helmet that form no part of the claimed design. 

CLAIM The ornamental design for a helmet, as shown and described. 